Isomerizing a c6 hydrocarbon fraction



Dec. 22, 1959 N. D. CARTER ET AL IsoMERIzING A cs HYDRocARBoN FRACTION Filed May 9, 1958 .United States Patent O ISOMERIZING A C8 HYDRDCARBON FRACTION Norman D. Carter, Poughkeepsie, and Howard V. Hess,

Glenham, N.Y., assignors to Texaco I-nc., a corporaon of Delaware Application May 9, 1958, Serial No. 734,327 7 Claims. (Cl. m50-683.65)

This invention relates to a method of treating a C6 hydrocarbon fraction. More particularly, this invention relates to a method of treating a C6 petroleum fraction such as produced in the fractionation of petroleum naphthas, reformates, etc. Still more particularly, this invention relates to an improved method of treating a C6 hydrocarbon fraction for the separation therefrom of a high octane material particularly useful as a blending agent for the manufacture of motor fuels.

ln the manufacture of high octane gasoline or components therefor it has been suggested that a C6 hydrocarbon fraction be subjected to isomerization for the production of a relatively high octane isomate. In the isomerization of a C6 hydrocarbon fraction the hydrocarbons generally found in equilibrium in the resulting isomate are listed in accompanying Table No. I, along with their octane numbers and boiling points.

As indicated in accompanying Table No. I, a substantial number of the components found in the isomerization reaction products resulting from the isomerization of a C8 hydrocarbon fraction have an octane number greater than 100. It is apparent, therefore, that if a C6 isomerization treating process could be carried out such that substantially only those components having an octane number greater than 100 were produced the resulting treated isomerization reaction products would be very valuable as a high octane gasoline or blending agent for the preparation of high octane gasoline;

Accordingly, it is an object of this invention to provide an improved process for the manufacture of high octane gasoline or high octane gasoline blending components from a C hydrocarbon fraction. r l Another object of this invention is to carry out a 'C5 isomerization process wherein substantially all of the `isomerized products recovered therefrom have an octane number of at least 100.

Another object of this invention is to provide an T'Yet' another `object of this invention is to provide a visomerization process wherein "nhexane and other "i Vicc relatively low octane number hydrocarbons are recycled to extinction.

AHow these and other objects of this invention are achieved will become apparent in the light of the accompanying disclosure and drawing which schematically illustrates a process in accordance with the practice of this invention.

In accordance with this invention an improved C6 isomerization reaction for the upgrading of a C6 hydrocarbon or petroleum fraction for the production of high octane motor fuels is obtained byseparately removing n-hexane and a mixture of 2methylpentane and S-methylpentane from the isomate and recycling these hydrocarbons to the isomerization reactionrthe isomerization reaction being carried out and the resulting isomate being treated such that only the relatively high octane hydrocarbons such as 2,2-dimethylbutane, 2,3-dimethylbutane, methylcyclopentane, benzene and cyclohexane are directly removed as products from the isomerization reaction effluent.

More particularly, in the practice of this invention the isomerization reaction eiiiuent resulting from the isomerization of a C6 hydrocarbon fraction such as a light virgin or straight run naphtha or a light reformate is subjected to operations which include a fractional distillation step for the separation of 2,2-dimethylbutane and 2,3- dimethylbutane therefrom and which also includes the separation of normal hexane from the isomerization reaction eilluent. In the practice of this invention n-hexane is removed from other components in the isomerization reaction eilluent by contacting the isomerization reaction effluent with an alumino-silicate molecular sieve type selective adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons. Following the above-indicated selective adsorption operation for the removal of straight chain hydrocarbons, i.e., n-hexane from the isomerization reaction etiiuent, there is produced an isomerization reaction efuent substantially free of nhexane.

In the practice of this invention any selective adsorption process effective for the removal of straight chain saturated hydrocarbons from branched chain hydrocarbons is satisfactorily employed in the practice of this invention. The invention, however, is particularly applicable to a selective adsorbent comprising certain natural or synthetic zeolites or alumino-silicates, such as a calcium alumino-silicate, which exhibit the property of a molecular sieve, that is, matter made up of porous crystals wherein the pores of the crystals are of molecular dimension and are of substantially uniform size. In general, zeolites may be described as water-containing alumino-silicates having a general formula (R,R2)O.AlgO3.nSiO2.mH2O wherein R may be an alkaline earth metal such as calcium, strontium or barium or magnesium and wherein R' is an alkali metal such as sodium or potassium or lithium. These materials when dehydrated retain their crystalline structure and are particularly useful as selective adsorbents for the selective adsorption of straight chain hydrocarbons.

A particularly suitable solid adsorbent for the adsorptive separation of straight chain hydrocarbons is a calcium alumino-silicate, apparently actually a sodium calcium aluminosilicate, manufactured by Linde Air Products Company and designated Linde Type 5A Molecular Sieve. The crystals of this particular calcium alumino-silicate have a pore size or opening of about 5 Angstrom units, a pore size suciently large to admit straight chain hydrocarbons to the substantial exclusion of the non-straight chain hydrocarbons, i.e., naphthenic, aromatic, isoparainic and isooleinic hydrocarbons. This particular selective adsorbent is available in various sizes, such as in the form 'of 1%" or '1/16" diameter pellets,

or as a finely divided powder having a particle size in the range 0.5-5.0 microns. A selective adsorbent employed in the practice of this invention may be in any suitable form or shape, granular, spheroidal or microspheroidal.

Particularly suitable selective adsorbents which may be employed in the practice of this invention include the synthetic and natural zeolites which, when ldehydrated, may be described as crystalline Zeolites-having a rigid three dimensional anionic'network and having interstitial dimensions sufficiently large to adsorb straight chain hydrocarbons but sufciently small to exclude thenonstraight chain hydrocarbons possessing larger'moleoular dimensions. The naturally occurring zeolite, chabazite, exhibits such desirable properties. Another suitable naturally occurring Zeolite is analcite NaAlSiZOGHgG which, when dehydrated and when all or part of the sodium is replaced by an alkaline earth metal such as calcium yields a material which may be represented by the formula (Ca,Na2)Al2Si4O12.H2O and which after suitable conditioning will adsorb straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons. Other naturaily occurring or synthetically prepared zeolites such as phacolite, gmerlinite, harmotome and the like or suitable base exchange modifications of these zeolites may also be employed in the practice of this invention.

Other solid inorganic or mineral selective adsorbents are known. lt is contemplated that selective adsorbents having the property of selectively adsorbing the straight chain hydrocarbons to the substantial exclusion of nonstraight chain hydrocarbons in the manner of a molecular sieve may be obtained by suitable treatment with various oxide gels of the polyvalent amphoteric metal oxides.

The selective adsorption of the n-hexane from the isomerization reaction effluent may be carried out in the liquid or gaseous phase and at any suitable temperature and pressure effective in the adsorptive separation operation. lt is desirable, however, to coordinate the adsorption separation conditions, e.g., temperature and pressure, with the desorption separation conditions, more fully described hereinafter, so as to effect the most economical use of the materials employed and for ease of control.

The adsorptive separation or adsorption of the nhexane from the isomerization reaction efuent by the selective adsorbent may be carried out at any suitable temperature, such as a temperature in the range 50500 F., suflicient to effect the adsorptive separation of the n-hexane by the. adsorbent,vand at any suitable pressure, such as a pressure in the range -l0,000 p.s.i.g. more `or less, the temperature and pressure being adjusted with respect to the isomerization reaction admixture undergoing treatment depending upon whether or not it is desired to maintain the mixture undergoing separation in the liquid phase or in the vapor or gaseous phase. Liquid phase adsorption may be carried out by simply slurrying the solid selective adsorbent with the liquid isomerization reaction mixture being treated, forlowed by separation or decantation of the resulting treated isomate, now substantially free of or having a substantially reduced n-hexane content. Liquid phase adsorption may also be carried out by percolating the liquid hydrocarbon mixture to be treated through a bed of solid adsorbent material.

lt is preferred, however, to carry out the adsorptive separation operation for the removal of n-hexane from the isomerization reaction effluent in the gaseous phase, that is, to maintain the'isomate mixture undergoing treatment in the vapor phase during the adsorption operation. In such an operation any suitable method for effecting vgas-solid contact may be employed, for example, a fixed bed, a moving bed, a lluidized bed or gas-entrained mass of selective adsorbent may be employed duringthe gas phaseadsorptive separation opera- V0-10,000 p.s.i.g., morev orv less.

tion. After sufficient time, the solid adsorbent is separated from the resulting treated isornate, now-substantially free of n-hexane, andthe resulting adsorbent, now substantially saturated with n-hexane, subsequently treated to desorb the adsorbed n-hexane therefrom.

The ydesorption of the n-hexane from the solid adsorbent material may be carried out at any suitable ternperature and pressure. For example, the desorption operation may be carried out ata temperature in the range It is preferred, however, that the `desorption operation be carried out in the gaseous phase, that is, the desorption or desorbing fluid, in the practice of this invention a gaseous mixture containing Z-methylpentane and `3-r`nethylpentane, being in the gaseous phase and the resulting desorbed n-hexane being present in the resulting desorption effluent in the gaseous or vapor phase. Accordingly, the desorption temperature and the desorption pressure are ad- Hjusted to maintain*the-desorbingliluid (Z-methylpentane vtion operation at a pressure substantially lower-than the adsorption pressure.

Isobaric adsorption-desorption operations are advantageous-in some instances;-however the pressure employed during the adsorptive separation operation is not determinative of the desorption pressure.

Any suitable desorption temperature suiciently high to elfect desorption of-the adsorbed n-hexane may be employed in the practice-of.thisinvention Usually a temperature in the'range 20C-80,0"` `F. is employed during the desorption operation, desirably a temperature above the critical temperature of n-hexane (455 F.). It is generally preferred to carry `out the-desorption operation at an elevated temperature in the range SOO-600 F. The desorption temperature employed, however, should not be excessively high, for example notgreater than 1100 F., particularly in the instance wherein a material s-uch as Linde Type 5A Molecular Sieve, that is, calcium alumino-silicate is employed as the selective adsorbent, since these relatively high temperatures are excessive and lead to the destruction of the adsorbent material, presumablyby collapse of the crystal structure With the resultant loss ofthe selective adsorption properties.

Although the desorption of n-hexane fromtheadsorbent containing the same may be accomplished bythe mere application of heat, or by use of a gaseous desorbing medium such as hydrogen,y which might be recovered from theisomerization reaction whichis desirably lcarried out in the presence of hydrogen or by means of a gaseous desorbing fluid such as isobutane which is readily recoverable by distillation .from the resulting desorbed n-hexane, it is preferred in the practice ofthis invention to employ a component or components separated from the isomerization reaction eiuent itself, viz. Z-methylpentane and/or B-methylpentane.

In the practice of this invention any suitable isomerization process effective for isomerizing n-hexane into its various corresponding C6 isomate, might be suitably employed. Suitable processes are known for effecting the isomerization of a C6 hydrocarbon such as n-hexane and generally employ. a precious.metal-containing catalyst such as a platinum-containing catalyst and are usually carried out in the gaseous or vapor phase. Suitable C6 isomerization processes are well known in the art, for example see The OilandGas Journal, March,25, 1957, pages 153-165. The disclosures of the aboveidentified publication with respect to isomerization processes suitableforuse in the. practice of this invention-are herein incorporatedrand. made party of this disclosure.

Other suitable isomerization processesfortheisomerization of C hydrocarbonfractions suchtasn-hexane are known and. are,suitably ,employed in. the practice, of this invention. As indicated hereinabove, the desorption of.

n-hexane from the selective adsorbent, particularly when an alumino-silicate molecular steve adsorbent is employed, is carried out substantially above the critical temperature of n-hexane (above about 454 F.). When the desorption operation is carried out at a temperature above the critical temperature of n-hexane substantially no hexane remains adsorbed in the adsorbent.

Referring now to the accompanying drawing which schematically illustrates a process in accordance with this invention and which is directed to the upgrading of a C6 hydrocarbon fraction containing a substantial amount, eg., at least about by vol. of n-hexane, together with isomeric C6 and other C5 hydrocarbons, a C6 hydrocarbon stream such as is derived from the distillation of crude oil, e.g., a light straight run naphtha in the C6 boiling range, or a C5 catalytic reformate fraction, is supplied via line 11 to isomerization reactor 12. Within isomerization reactor 12 the C6 hydrocarbon fraction undergoes isomerization, desirably in the presence of gaseous hydrogen, with the resulting production of an isomerization reaction eluent or isomate which is recovered therefrom via line 14 and passed to isomate fractionator 15. The isomate leaving isomerization reactor 12 via line 14 will comprise the following components in admixture; 2,2-dimethylbutane, 2,3-dimethylbutane, Z-methylpentane, l-methylpentane,` n-hexane, methylcyclopentane, benzene and cyclohexane.

Within isomate fractionator 15 the isomerization reaction efuent is fractionated and there is recovered overhead via line 16 an admixture comprising substantially all of the 2,2-dimethylbutane and 2,3-dimethylbutane present in the isomerization reaction eluent. At the same time there is recovered as a bottoms fraction from isomate fractionator 15 via line 18 an admixture comprising 2methylpentane, 3methylpentane, n-hexane, methylcyclopentane, benzene and cyclohexane. The bottoms fraction recovered from isomate fractionator 1.5 is supplied via line 18 to adsorber 19 wherein it contacts an alumino-silicate molecular sieve type selective adsorbent which selectively adsorbs the n-hexane therein to the exclusion of the other hydrocarbons.

There is recovered from adsorber 19 via line 20 a resulting treated eflluent comprising 2-rnethylpentane, 3- methylpentane, methylcyclopentane, benzene and cyclohexane and substantially no n-hexane. The resulting treated eluent is then sent via line 20 to adsorber effluent fractionator 2l wherein there is recovered an overhead fraction comprising substantially all of the Z-methylpentane and 3-methylpentane in the adsorber effluent and a bottoms fraction recovered from adsorber eiuent fractionator 21 via line 22 comprising methylcyclohexane, benzene and cyclohexane.

The fraction recovered overhead from fractionator 21 via line 24 and containing substantially all of the 2- methylpentane and 3-methylpentane in the isomerization reaction eluent is employed to effect desorption of the adsorbed n--hexane from the selective adsorbent employed.

As illustrated in the accompanying drawing while one adsorber, adsorber 19, is employed in the adsorptive separation stage in the practice of this invention for the separation of n-hexane from the isomate, another adsorber, adsorber 25, is undergoing the desorption stage for the desorption of the n-hexane from adsorbent previously employed in the adsorptive separation of n-liexane from the isomate. As a result of the desorption operation carried out in adsorber 25 there issues from adsorber 25 via line 26 a desorption efuent comprising Z-methylpentane, S-methylpentane and the n-hexane originally present in the isomate. The resulting desorption eluent comprising substantially only these three components is then recycled via line 26 through line 11 to isomerization reactor l2.

Although in the practice of this invention, as illustrated in the accompanying drawings, the selective adsorption of n-hexane from the isomate is shown as having been carried out subsequent to the fractional distillation of the isomate for the separation of 2,2-dimethylbutane and 2,3-dimethylbutane, if desired the separation of n-hexane from the isomate mixture may be carried out prior to the separation of 2,2-dirnethylbutane and 2,3-dimethylbutane therefrom by lfractional distillation. Also, if desired the C5 hydrocarbon fraction undergoing treatment in accordance with this invention prior to being supplied to the isomerization reaction may, if desired, be subjected to a selective adsorption operation for the separation of n-heXane therefrom and4 to a fractional distillation step for the removal of 2,2-dimethylbutane and 2,3-dimethylbutane therefrom and for the recovery of Z-methylpentane and `3methylpentane therefrom. The thus-separated 2,2-dimethylbutane and 2,3-dimethylbutane then separately recovered as product and the separated n-hexane desorbed from the adsorbent by contact with Z-methylpentane and B-methylpentane and the resulting desorption efuent supplied as substantially the only feed to the isomerization reactor.

In the accompanying disclosure and drawing no reference has been made to conventional operating accessory equipment such as heat exchangers, pumps, instruments, tlow regulating devices, liquid level regulating devies and the like for reasons of clarity and so as not to clutter up the specification and drawing. The employment of such devices, equipment and instruments in an operation in accordance with this invention is readily apparent to those skilled in the art in the light of this disclosure. Further, it is evident from the foregoing disclosure that many modifications, substitutions and alterations are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

1. A method of treating a hydrocarbon fraction containing a major amount of C6 hydrocarbons which comprises subjecting said fraction to isomerization to yield an isomate, subjecting said isomate to fractionation to separate therefrom substantially all of the 2,2-dimethylbutane and 2,3-dimethylbutane in said isomate, subjecting the remaining isomate to contact with an aluminosilicate molecular sieve adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of non-straight chain hydrocarbons to separate n-hexane therefrom and to yield a treated efuent substantially free of n-hexane, subjecting said treated euent to fractionation to separate therefrom a fraction consisting essentially of Z-methylpentane and 3-methylpentane, employing the thus-separated Z-methylpentane and 3-methylpentane to desorb the adsorbed n-hexane from said adsorbent and returning the desorption elfluent resulting from the desorption of n-hexane from the adsorbent by contact with 2-methylpentane and 3-methylpentane to the isomerization reaction in admixture with the CB hydrocarbon fraction undergoing treatment.

2. A method in accordance with claim l wherein said isomerization reaction is carried out in the presence of a platinum-containing catalyst.

3. A method in accordance with claim 1 wherein said C6 hydrocarbon fraction prior to isomerization is subjected to fractionation for the removal of 2,2-dimethylbutane and 2,3-dimethylbutane therefrom prior to subjecting the C6 hydrocarbon fraction to isomerization.

4. A method in accordance with claim l wherein said C6 hydrocarbon fraction is subjected to contact with said selective adsorbent for the removal of n-hexane therefrom and the resulting treated C6 hydrocarbon fraction, now substantially free of n-hexane, fractionated for the removal of 2,2-dimethylbutane and 2,3-dimethylbutane therefrom and the remaining C6 fraction fractionated for the separation of the 2-methylpentane and S-methylpentane components therefrom, which components are then employed to desorb n-hexane from said adsorbent and the resulting desorption effluent passed to the isomerization reaction.

5. A method of treating a C6 hydrocarbon fraction which comprises subjecting said fraction to isomerization to yield an isomate mixture comprising 2,2-dimethylbutane, 2,3-dimethylbutane, n-hexane, 2-methylpentane, 3-methylpentane, methylcyclopentane, benzene and cyclohexane, subjecting said isomate mixture to fractional distillation to separate overhead a fraction consisting essentially of 2,2-dimethylbutane and 2,3-dimethylbutane, recovering from the aforesaid fractionation operation a bottoms fraction consisting essentially of 2-methylpentane, 3-methylpentane, n-hexane, methylcyclopentane, Ibenzene and cyclohexane, subjecting said bottoms fraction to contact with an alumino-silicate molecular sieve selective adsorbent which selectively adsorbs straight chain hydrocarbons to the substantial exclusion of nonstraight chain hydrocarbons to adsorb n-hexane therefrom, recovering from the aforesaid selective adsorption operation an effluent consisting essentially of 2methyl pentane, 3-methylpentane, methylcyclopentane, benzene and cyclohexane, subjecting said eflluent to fractionatio-n to recover overhead a fraction containing substantially only Z-methylpentane and 3-methylpentane and to yield a bottoms fraction containing substantially only methylcyclopentane, pentane and cyclohexane, employing said overhead fraction containing Z-methylpentane and 3-rnethylpentane to desorb the adsorbed n-hexane from said adsorbent and returning the resulting desorption effluent consisting essentially of 2-methylpentane, 3methylpentane and n-hexane to the aforesaid isomerization reaction.

Cil

I6. Amrethod of treating a C6 hydrocarbon fraction which comprises subjectingwsail `fraction to isomerization to yield an `isomatve mixture consisting essentially of 2,2-dimethylbutane, 2,3v-dirnethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, methylcyclopentane, benzene and cyclohexane, subjecting said isornate to contact with an alumino-silicate molecular sieve adsorbent for the removal of n-hexane therefrom, fractionating the resulting treated effluent to separate therefrom a relatively low boiling fraction consisting essentially of 2,2-dimethylbutane and 2,3-dimethylbutane, an intermediate boiling fraction consisting essentially of 2-rnethylpentane and 3-methylpentane and a relatively high boiling fraction consisting lessentially of methylcyclopentane, benzene and cyclohexane, employing said intermediate fraction to desorb the adsorbed n-hexane from said adsorbent and returning the resulting desorption eyluent consisting essentially ofZ-methylpentane,4 3-methylpentane and n-hexane to the aforesaid isomerization reaction.

7. A method in accordance with claim 6 wherein the fractions containing 2g2-dimethylbutane, 2,3-dimethylbutane, metbylcyclopentane, benzene and cyclohexane are recovered and blended to yield a high octane motor fuel component.

References Cited in the tiler ofjthis patent UNITED STATES rPATENTS 2,395,022 Sutton et al. Feb. 19, 1946 2,396,331 Marschner Mar. 12, 1946 2,766,302 Elkins Oct. 9, 1956 2,818,455 Ballard et al. Dec. 31, 1957 

1. A METHOD OF TREATING A HYDROCARBON FRACTION CONTAINING A MAJOR AMOUNT OF C6 HYDROCARBONS WHICH COMPRISES SUBJECTING SAID FRACTION TO ISOMERIZATION TO YIELD AN ISOMATE, SUBJECTING AID ISOMATE TO FRACTIONATION TO SEPARATE THEREFROM SUBSTANTIALLY ALL OF THE 2,2-DIMETHYLBUTANE AND 2,3-DIMETHYLBUTANE IN SAID ISOMATE, SUBJECTING THE REMAINING ISOMATE TO CONTACT WITH AN ALUMINO-SILICATE MOLECULAR SIEVE ADSORBENT WHICH SELECTIVELY ADSORB STRAIGHT CHAIN HYDROCARBONS TO THE SUBSTANTIAL EXCLUSION OF NON-STRAIGHT CHAIN HYDROCARBONS TO SEPARATE N-HEXANE THEREFROM AND TO YIELD A TREATED EFFLUENT STANTIALLY FREE OF N-HEXANE,SUBJECTING SAID TREATED EFFLUENT TO FRACTIONATION TO SEPARATE THEREFROM A FRACTION CONSISTING ESSENTIALLY OF 2-METHYLPENTANE AND 3-METHYLPENTANE, EMPLOYING THE THUS-SEPARATED 2 -METHYLPENTANE AND 3-METHYLPENTANT TO DESORB THE ADSORBED N-HEXANE FROM SAID ADSORBENT AND RETURNING THE DESORPTION EFFLUENT RESULTING FROM THE DESORPTION OF N-HEXANE FROM THE ADSORBENT BY CONTACT WITH 2-METHYLPENTANE AND 3-METHYLPENTANE TO THE ISOMERIZATION REACTION IN ADMIXTURE WITH THE C6 HYDROCARBON FRACTION UNDERGOING TREATMENT. 